釋氏要覽 1.98

1.98

史云。首座之名。即上座也。居席之端。處僧之上故也。即唐宣宗署僧辯章。為三教首座。此為始也今則以經論學署首座也。

The history states: "The designation 'head seat' means the elder (sthavira). It is because he occupies the head of the seats and sits above the saṃgha. Emperor Xuānzong of the Tang appointed the monk Bianzhang as Head Seat of the Three Teachings, which was the beginning of this. Nowadays, one is appointed head seat based on learning in scriptures and treatises."